Spinal prostheses, implants, and fixation systems are routinely coupled to a portion of a spinal column to treat various conditions. Such procedures often employ one or more screws or similar hardware to anchor and/or secure a portion of a prosthesis or fixation structure, such as a rod assembly or the like. Important considerations for the implantation of spinal instrumentation include the ability to provide safe insertion, rigid fixation, and ease and adaptability of implantation.
It has been identified, however, that considerable difficulties may be associated with inserting rigid screws with a dynamic system to stabilize a spinal segment, while simultaneously positioning the prostheses or implants such that they are aligned to engage the screw without distortion and without shear stresses. Attempts at achieving proper variability of the screw/implant interface having limited maneuverability may require additional connectors and increased operating time, which subsequently may enhance many complications associated with surgery. Often, desired surgical results with such limited devices cannot be achieved, thereby rendering such instrumentation attempts entirely unsuccessful.
While a variety of attempts have been made at providing instrumentation which permit some freedom with respect to angulation of the screw and the coupling portion of an implant, these devices are generally complex, inadequately reliable, lack long-term durability, and fail to provide the freedom needed for dynamic motion. These considerable drawbacks associated with prior art systems also include difficulty in properly positioning an implant for engagement, and the tedious manipulation of the many parts that are used in the prior art to lock the rod, the screw, and the coupling element in position once they are properly aligned. Moreover, displacement of the screw and/or a portion of an implant or prostheses may occur as these parts are manipulated to securely couple these components together.
In addition, a problem often encountered with the implantation and/or affixation of orthopedic implants or instruments is that, over time, the fixation screws used to secure an implant and/or device to a portion of a bone tend to back out or loosen with time, especially if placed within an area experiencing regular extension, flexion and other movement. Such loosening, while sometimes harmless, can lead to pain and failure of the device or implant. Moreover, screw loosening or screw migration can have serious consequences in the case of spine fixation, where a loose screw can puncture or otherwise damage surrounding tissue, as well as failing to structurally support the intended portion of the spine.
Dynamic stabilization of the spine represents a modern concept in spinal fixation where a posterior pedicle screw-based device substantially controls the motion of the spinal segment. Such dynamic stabilization devices are often anchored to pedicle screws, and as the system is not particularly rigid, there may be increased stresses transmitted to the pedicle screw, resulting in a greater likelihood that the screw will loosen and/or migrate. Moreover, a dynamic stabilization system may include an artificial lumbar disc, where motion of a posterior stabilizer corresponds and/or complements the motion of the disc. In such a system, a portion of the movement and/or range of motion of the system can be accommodated by the pedicle screw itself in combination with the posterior stabilizer.
In view of the above, it would be desirable to provide a fixation assembly having a desired degree of freedom of angulation and rotation with respect to a portion of an implant or prosthesis or the portion of the screw embedded in the bone, and further provide for expeditious implantation. It is further desirable to provide an implant system that provides a means of locking the fixation screw into the implant and to prevent loosening or screw migration. In addition, the need exists for such a system to be provided in implants suitable for small surgical areas, such as certain spinal regions, which is reliable, durable, and thereby provides long term fixation support. It would also be desirable to provide a fixation assembly that resists loosening by providing a range of motion between a head portion and a shaft, such that forces experienced by the fixation assembly are dissipated through controlled motion. It would also be desirable for a fixation assembly to include a desired degree of motion to allow for changes needed in orientation of an implant that are requisite for dynamic stabilization devices.